In any cellular system, it is of very high importance that battery powered, mobile nodes (hereafter referred to as “user equipments,” or “UEs”) can spend most of their time in a low activity state to preserve energy. Typically, a cellular system will have one or more defined “active” modes, where the UE is controlled by the network and is instructed to attach to a certain cell, do certain measurements etc. The system will generally also have one or more “idle” or “dormant” modes, where the UE typically listens only to certain signals from the network and makes its own decisions regarding which cell or cells to listen to, and when to report back status updates.
Most UEs in most cellular systems today spend a majority of their time in dormant mode, and therefore it is of utmost importance that the UEs can consume as little power as possible in dormant mode.
In a cellular system like as the 5th-generation radio access network (RAN) currently being defined by the 3rd-Generation Partnership (3GPP) and often referred to as “New Radio” or “NR,” beamforming can be used for the transmission of cell information signals. “Beamforming” here refers to a (usually) highly directional transmission of the signal energy for a given signal or set of signals, e.g., with 3-dB beam-widths of less, often substantially less, than 90 degrees in the horizontal plane, for downlink transmissions. While conventional transmissions are shaped to some degree, e.g., to avoid transmitting excessive energy in a vertical direction and/or to direct the majority of the signal energy to a particular cell sector, the beamformed transmissions discussed herein are intentionally shaped to a greater extent, so that, for example, any given downlink beam provides useful signal strengths only within a small fraction of the area that is generally served by the transmitting node. Accordingly, to serve the entire area, the transmitting node may make use of multiple, and perhaps very many, beams, which may be time-multiplexed, frequency-multiplexed, or both.
Beamforming cell information signals or broadcast signals, such as so-called mobility reference symbols, rather than transmitting them over an entire cell, may be done for several reasons. One reason is to increase the effective antenna gain of the transmitter, e.g., to compensate for higher path loss in high frequency bands or to enable extended coverage at traditional frequencies. Another reason is to obtain a rough spatial positioning of a UE, based on the directionality of the beam.
Typically, the beamformed cell information signals will be time multiplexed between beams so that high output power can be used for each beam.